1. Field of the Invention
The present invention relates to an image processing method, to a profile generation method, and to an image processing apparatus. More particularly, the present invention relates to gamut mapping used in a color conversion profile.
2. Description of the Related Art
Recently, digital devices, such as a digital camera and an image scanner, have become widespread. Thus, digital images can readily be obtained. Also, a full-color hard copy technology has rapidly developed. Especially, concerning an inkjet printing technology, a print-quality level has been comparable to that of silver-halide photographs. Thus, the inkjet printing technology has become widely used. Meanwhile, networks, such as the Internet, have become widespread. Many users are in an environment in which various devices can be connected to the networks. In such an environment in which various input-output (I/O) devices are used, color image data is input and output among devices differing in color reproduction range (color gamut) from one another, for example, in a case where a hard copy of a color image signal of a monitor having a certain color reproduction range is produced by a printer whose color reproduction range is narrower than that of the monitor.
Meanwhile, a “color management system” (hereunder referred to as “CMS”) is known as a technique of performing color reproduction of a same color among different devices.
FIG. 1 illustrates an exemplary configuration of the CMS. FIG. 1 shows the CMS using a device-independent color space. For example, in a case where an image input device, such as a camera or a scanner shown in FIG. 1, is connected to an image output device, such as a printer or a monitor shown in FIG. 1, conversion between a color signal of an image input device and a color signal of an image output device is realized by interposing a profile group therebetween.
Each of profiles includes a transformation formula representing the relation between each device color and a device-independent color space, or includes a transformation table generated in advance as a lookup table (LUT) representing the transformation. Thus, the color conversion can be performed through the device-independent color space (for example, CIE-XYZ or CIE-L*a*b*) by using the profiles.
This system has an advantage in that image data exchange can easily be performed among systems differing from one another in input and output devices connected thereamong.
According to the CMS, a color, which can be reproduced by a certain input device, is reproduced by an output device. Thus, the technique of gamut mapping, which absorbs differences in color reproduction range among the input and output devices, is used.
Generally, individual devices differ in color gamut from one another. For example, a monitor performs color reproduction utilizing additive mixture of three primary colors, that is, red (R), green (G), and blue (B) by color development of fluorescent materials respectively corresponding to the three primary colors. Therefore, the color gamut of the monitor depends upon the kind of fluorescent materials used. Meanwhile, the color gamut of a printer varies not only with a kind of ink used but with the kind of paper used.
FIG. 2 is a graph illustrating the relation between the color gamut of a monitor's sRGB color space and that of an inkjet printer and the relation between lightness and saturation in the case of a certain hue.
In a case where the printer's color gamut 202 is smaller than the monitor's color gamut 201 as shown in FIG. 2, when a color is within the monitor's color gamut and is outside the printer's color gamut, the color cannot be reproduced by the printer.
Therefore, in such a case, it is necessary to perform mapping of a color, which is outside of the printer's color gamut, into this gamut while original image information is maintained as much as possible. Generally, mapping of a color, which cannot physically be reproduced by a device, into a color gamut of the device by some kind of processing is referred to as “gamut compression” (or “gamut mapping”).
Generally, a maximum lightness value 201H and a minimum lightness value 201L in a monitor's color gamut, which is a mapping source, are respectively different from a maximum lightness value 202H and a minimum lightness value 202L in a printer's color gamut that is a mapping destination. Although depending upon a mapping method used in the gamut compression, in this case, natural color reproduction can usually be achieved in a high saturation region of a gamut-compression result. However, in a low saturation region thereof, tonability may be degraded in a low saturation region of an image, which is output from the printer, due to the difference between the maximum lightness value and the minimum lightness value.
Hitherto, there has been known a method of performing gamut compression using a lightness value (hereunder referred to as a relative lightness value) L*std obtained by normalizing a lightness value L*in in a printer's color gamut, which is a mapping destination, to a lightness value L*ori in a monitor's color gamut, which is a mapping source. That is, the relative lightness value L*std is obtained by normalizing the lightness value L*w, which corresponds to a white color of printer paper, to 100 degrees and also normalizing the lightness value L*bk, which corresponds to a black color of the printer paper, to 0. Thus, the relative lightness L*std is defined by the following equation (1).L*std=(L*in−L*bk)/(L*w−L*bk)×100  (1)
The lightness value L*w of the white color of the printer paper is a lightness value of a print medium, such as paper used by the printer. More specifically, the lightness value L*w is obtained by measuring a lightness value of the medium through the use of a calorimeter. The lightness value L*bk of the black color of the printer paper is a lightness value in the L*a*b* space obtained by causing the printer to output a patch, whose image is input to the printer and represented by R, G, B signals respectively having signal levels (R, G, B)=(0, 0, 0), and then performing color measurement on the patch through the use of a calorimeter.
Thus, the degradation of the tonability in the low saturation region can be suppressed by normalizing the lightness value in the color gamut of the mapping destination to the lightness value in the color gamut of the mapping source. However, the problem due to compression mapping cannot appropriately be solved only by such normalization of the lightness value.
Compression mapping also has a problem in that a tonal balance is impaired by the mapping.
In a mapping method shown in FIG. 2 in which points (P1, P2, . . . ), which are outside the printer's color gamut 202 and are within the monitor's color gamut 201, are compressed to a single point T (that is, a convergence point), the tonal balance may be impaired in an image displayed on the screen of the monitor. That is, in the example shown in FIG. 2, a rate of a change from the point P1 to the point P2 in the monitor's color gamut to the entire image displayed on the screen of the monitor is substantially equal to a rate of a change from a point P1′ to a point P2′, to which the point P1 and P2 are respectively mapped, in the printer's color gamut to the entire image printed on the printer paper. Thus, there is good tonal balance. However, depending on the relation between the monitor's color gamut and the printer's color gamut, when the compression mapping of compressing the points in the mapping source to a single point in the mapping destination is performed, a tonal change rate may notably be varied before and after the mapping. In this case, the image output by the printer differs in tonability from the image displayed on the screen of the monitor.
Japanese Patent Application Laid-Open No. 9-163170 describes the technique of finding a highest saturation point in an output-device's color gamut corresponding to each hue and subsequently performing compression mapping by setting a point, the corresponding lightness value of which is equal to that of the highest saturation point, on the saturation axis as a convergence point corresponding to each hue. According to this method, a convergence point can be determined in accordance with the shape of the gamut corresponding to each hue. Also, the tonal balance can be suppressed from being lost.
However, the simple compression-mapping technique employing a single convergence point has a problem in that this technique cannot prevent occurrence of reversal of a tonal change, such as a saturation change. As illustrated in FIG. 2, when the location is changed from the point P1 to the point P2 in the monitor's color gamut 201, the saturation is reduced. Conversely, when the location is changed from the point P1′ to the point P2′, to which the points P1 and P2 are respectively mapped, in the printer's color gamut 202, the saturation is increased. Thus, when the compression-mapping technique employing a single convergence point is performed, the reversal of various tonal changes, such as a saturation change, inevitably appears.
The reversal of various tonal changes, such as a saturation change, can be prevented by appropriately changing the convergence point of the gamut compression corresponding to each of the color gamut of the mapping source. However, in this case, the adaptive setting of the convergence point is needed. Consequently, processing is complicated.